DNA Replication...2. DNA replication was postulated by Watson and Crick after they dicoverd the...

Dr. Shatha S. Jumaah/Lecturer _____________________________________________ Genetics – 2nd Grade /1st Semester

shtaha.saadi@tiu.edu.iq https://tiu.edu.iq/

2020 - 2021

TIU - Faculty of Science Medical Analysis Department

DNA Replication

DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule DNA replication,

also known as semi-conservative replication, is the process by which DNA is essentially doubled.

1. In molecular biology DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule .

2. DNA replication was postulated by Watson and Crick after they dicoverd the structure of DNA .

3. DNA replication is semiconservative . Each srand in the double helix

acts as a template for synthesis of a new , complementary . 4. The process of DNA replication is vital for cell growth , repair , and

reproduction in organisms .

1.DNA replication is the process by which DNA makes a copy of itself during cell division .

2.This process takes us from one starting molecule to two daughter molecules , with each newly formed double helix containing one

new and one old strand . 3.DNA found within the nucleus , must be replicated in order to

ensure that each new cell receives he correct number of chromosomes. The process of duplication is called DNA replication .

4.Fundamental process occurring in all cells for copying DNA to

transfer the genetic information to daughter cells .

What is replication of DNA?

1 . Replication is always semiconservative .

2 . Replication begins at the sequences called origins .

3 . DNA synthesis is initiated by short fragments of RNA call primers .

4 . The elongation of DNA strands is always in the 5’ to 3’ direction .

5.DNAreplicationcanbeuniorbidirectional.

6 . Replication is continuous on the leading strand and discontinuous on the lagging strand .

7 . New nucleotide strands are complementary and antiparallel to their template strands .

8 . Replication takes place at very high rates and is astonishingly accurate due to the processes of nucleotide selection , proof reading and repair mechanisms .

Before DNA can be replicated the double stranded molecule must be unzipped into two single strands . DNA has four base called adenine , thymine , cytosine and guanine that form pairs between the two strands . In order to unwind DNA these interaction between base pairs must be broken. These is performed by an enzyme known as DNA helicase . DNA helicase separate the strands into Y shape known as the replication fork .

Step 2 :- Primer binding

The leding strands is the simplest to replicated . once the DNA strands have been separated , a short piece of RNA

called a primer binds to the 3’ end of the strand . The primer always binds as the starting point for replication .

Primers are generated by the enzyme DNA primase . DNA polymerase 3’ can only nucleotides to existing strands of DNA .

Step 3 :- Elongation

Enzymes known as DNA polymerases are responsible creating the new

strands by a process called elongation .

In eukaryotic cells polymerases alpha , delta and epsilon are the primary

polymerases involved in DNA replication . Because replication proceeds in the 5’ to 3’ direction on the leading strand , the newly formed strands is

continuous . The lagging strands begins replication by binding with multiple primer .

Each primer is only several bases apart .

DNA polymerase then adds pieces of DNA , called Okazaki fragments , to

the strand between primers .

This process of replication is discontinuous as the newly created fragments

are disjointed .

Involves the addition of new nucleotides based on complementarity of the

templated strands .

The daughter strands is elongated with the binding of more DNA

nucleotides .

Step 4 :- Termination

Once both the continuous and discontinuous strands are formed , an

enzyme called exonuclease removes all RNA primers from the original strands . These primers are then replaced with appropriate bases . another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strands . The ends of the linear DNA presents a problem as DNA polymerase can only

add nucleotides in the 5’ to 3’ direction . Once completed the parent strand and its complementary DNA strands

coils into the familiar double helix shape . In the end replication produces two DNA molecules , each with one strand

from the parent molecule and one new strand .

1.DNA helicase :- unwind and separates double stranded as it moves along the

DNA It forms the replication fork by breaking hydrogen bonds between

nucleotide pairs in DNA.

2.DNA primase :- A type of RNA polymerase that generates RNA primers . RNA

molecule acts as templates for the starting point of DNA replication.

3. DNA polymerases :- Synthesize new DNA molecules by adding nucleotides to

leading and lagging DNA strands .

4. DNA Gyrase or (Topoisomerase) :- unwind and rewinds DNA strands to

prevent the DNA from becoming tangled or supercoiled .

5. DNA ligase :- joins DNA fragments together by forming phosphodiester

bonds between nucleotides .

6. Single strand binding proteins :- Keep the DNA single stranded after it has

been melted by helicase .

7. RNA primer :- RNA primer composed of multiple bases that attached to the

template strands to initiate the DNA replication .

8. Telomerase :- Finishes off the ends of DNA strands .

9. Exonucleases :- Group of enzymes that remove nucleotide bases from the

end of a DNA chain.

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